1. Field of the Invention
The present invention relates to a display device such as a liquid crystal display and an organic electroluminescent display and a display device manufacturing method. In particular, the present invention relates to a technique for improving optical characteristics of opening regions of a display device including pixels disposed in a matrix on a transparent base plate (substrate), each pixel having an opening region, in which an electro-optic element such as a liquid crystal cell and an organic electroluminescent element is formed, and a non-opening region, in which a thin film transistor for driving the electro-optic element is formed.
2. Description of the Related Art
FIG. 13 is a schematic partial sectional view showing a partial section of an example of a conventional active matrix type liquid crystal display device.
As shown in FIG. 13, a metal gate electrode 2 is formed on a base plate (substrate) 1 made of glass or the like. A gate insulation film 3 is formed so as to cover the gate electrode 2. On the gate insulation film 3, an amolphous semiconductor thin film 4A, which operates as an active layer of a thin film transistor, is formed. On one end side of the semiconductor thin film 4A, a drain electrode 5D is formed with an amolphous semiconductor thin film 4A(n+), which has a high impurity concentration and is made to have low resistance, inserted between the drain electrode 5D and the amolphous semiconductor thin film 4A. On the other end side of the amolphous semiconductor thin film 4A, a source electrode 5S is formed with another amolphous semiconductor thin film 4A(n+), which is also made to have low resistance, inserted between the source electrode 5S and the amolphous semiconductor thin film 4A. A protection film 8 is formed so as to cover the drain electrode 5D and the source electrode 5S. On the protection film 8, a pixel electrode 10 which comprises a transparent conductive film such as a film including indium tin oxide (ITO) as its main ingredient is formed to connect electrically with the drain electrode 5D through a contact hole CON.
The thin film transistor which is shown in FIG. 13 has a typical form of a bottom gate structure in which the amorphous semiconductor thin film 4A made of amorphous silicon or the like is used as its active layer. A thin film transistor having such a structure is called as a xe2x80x9creverse stagger channel etch type transistorxe2x80x9d.
FIG. 14 is a schematic partial sectional view showing a partial section of another example of a conventional display device. In FIG. 14, components corresponding to those of the prior art shown in FIG. 13 are designated by reference numerals corresponding to those in FIG. 13 for facilitating the understanding of them. Incidentally, in the following drawings, too, components corresponding to those shown in FIG. 13 are designated by reference numerals corresponding to those in FIG. 13 for facilitating the understanding of them.
The display device shown in FIG. 14 has basically the same structure as that shown in FIG. 13. However, the display device shown in FIG. 14 differs from the one shown in FIG. 13 in that a channel protection film 6 is formed on an amolphous semiconductor thin film 4A that functions as an active layer. The channel protection film 6 protects a part corresponding to a channel region of the active layer, which exists right above the gate electrode 2. The structure is called as a xe2x80x9creverse stagger channel protection type transistorxe2x80x9d.
FIG. 15 is a schematic partial sectional view showing a further example of a conventional display device.
A shading film 11 is formed on a base plate 1, and an amolphous semiconductor thin film 4A is formed above the shading film 11 with an undercoat film 12 inserted between the amolphous semiconductor thin film 4A and the shading film 11. A pixel electrode 10 is connected with one end of the semiconductor thin film 4A with an amolphous semiconductor thin film 4A(n+), which is made to have low resistance, inserted between the semiconductor thin film 4A and the pixel electrode 10. A source electrode 5S is connected with the other end of the semiconductor thin film 4A, similarly with an amolphous semiconductor thin film 4A(n+) inserted between the semiconductor thin film 4A and the source electrode 5S. The amolphous semiconductor thin film 4A, which functions as an active layer, is covered with a channel protection film 6 and a gate insulation film 3, and further a gate electrode 2 is formed on the gate insulation film 3.
The structure is the reverse of the prior arts described before in vertical positions of the amolphous semiconductor thin film 4A and the gate electrode 2, and then is called as a xe2x80x9cforward stagger transistorxe2x80x9d.
FIG. 16 shows an improved type of the prior art shown in FIG. 13.
In the structure shown in FIG. 16, a thin film transistor is covered with a leveling film 9, and a pixel electrode 10 is formed on the leveling film 9. The structure is a xe2x80x9chigh numerical aperture type transistor using a leveling filmxe2x80x9d.
A silicon nitride film or a silicon oxide film is frequently used as the gate insulation film 3 or the protection film 8 in the aforesaid prior art. Moreover, an organic resin film is frequently used as the leveling film 9.
The structures shown in FIGS. 13-16 are described in detail in, for example, xe2x80x9cAn Introduction to Liquid Crystal Display Engineeringxe2x80x9d, The Nikkan Kogyo Shimbun, Ltd., 1998, pp. 27-30, xe2x80x9cThe Latest Liquid Crystal Process Technique in ""99xe2x80x9d, Press Journal Inc., 1998, pp. 21-27, and xe2x80x9cFlat Panel Display 1999xe2x80x9d, Nikkei Business Publications, Inc., 1998, pp. 118-131.
The aforesaid display devices use an amolphous semiconductor thin film as their active layers, however the prior art shown in FIG. 17 uses a polycrystalline semiconductor thin film such as a film including polysilicon as the main ingredient as its active layer.
A gate electrode 2 is formed on a glass base plate 1, and a polycrystalline semiconductor thin film 4P is formed above the gate electrode 2 with a gate insulation film 3 inserted between the polycrystalline semiconductor thin film 4P and the gate electrode 2. A part of the polycrystalline semiconductor thin film 4P placed right above the gate electrode 2 is formed as a channel region, and parts on both sides of the channel region are formed as a source region S and a drain region D, where impurities are injected in a high concentration. The semiconductor thin film 4P is covered with an interlayer insulation film 7, and a drain electrode 5D and a source electrode 5S are formed on the interlayer insulation film 7. These electrodes 5D and 5S are covered with a protection film 8.
Such a structure is called as a xe2x80x9cbottom gate type transistorxe2x80x9d because the gate electrode 2 is disposed below the active layer.
FIG. 18 shows a structure of a thin film transistor using a polycrystalline semiconductor thin film as its active layer in the same way as the above example. In the structure of this prior art, differently from the structure shown in FIG. 17, a gate electrode 2 is formed above a polycrystalline semiconductor thin film 4P with a gate insulation film 3 inserted between the gate electrode 2 and the semiconductor thin film 4P. Such a structure is called as a xe2x80x9ctop gate type transistorxe2x80x9d.
FIG. 19 shows a CMOS structure in which a top gate structure N-channel thin film transistor (N-channel TFT) is combined with a P-channel thin film transistor (P-channel TFT). The P-channel thin film transistor uses a polycrystalline semiconductor thin film 4 as an active layer, in which, for example, boron is injected into a source region S and a drain region D. The N-channel thin film transistor uses a polycrystalline semiconductor thin film 4 as an active layer, in which phosphorus or the like is injected into a source region S and a drain region D.
In the example, the N-channel thin film transistor is used for driving a pixel electrode 10. As for this structure, for suppressing a leak current, the so-called lightly doped drain (LDD) structure may be employed by forming regions, into which impurities are injected in a low concentration, between the drain region D and a central channel region and between the source region S and the central channel region.
These thin film transistors using polycrystalline semiconductor thin films as their active layers are described in detail in, for example, xe2x80x9cThe Latest Liquid Crystal Process Technique in ""99xe2x80x9d, Press Journal Inc., 1998, pp. 53-59, and xe2x80x9cFlat Panel Display 1999xe2x80x9d, Nikkei Business Publications, Inc., 1998, pp. 132-139. In any cases, a silicon nitride film or a silicon oxide film is frequently used as a gate insulation film, an interlayer insulation film and a protection film, all of which constituting a thin film transistor.
In a conventional active matrix type liquid crystal display, a pixel has an opening region including a pixel electrode 10 formed with a transparent conductive film and a non-opening region, where a thin film transistor for driving the pixel electrode is formed. The non-opening region has a film structure, which contains a thin film transistor, composed of a gate insulation film, an interlayer insulation film, a protection film and the like. The film structure extends to the opening region as it is, and the film structure is laid between the pixel electrode 10 and a glass base plate 1. The film structure includes a silicon nitride film, a silicon oxide film, an organic resin film, and the like. Although the refractive index of the silicon nitride film is within a range of 1.8 to 2.0, the refractive indices of the glass base plate, the silicon oxide film and the organic resin film are within a range of about 1.4 to 1.6. Consequently, in a case where these films having different refractive indices are configured into a multilayer structure, optical interference effects occur at interfaces.
FIG. 20 shows a spectrum of transmissivity of a film body in which a multilayer structure composed of a silicon nitride film, a silicon oxide film or the like is formed on a glass base plate. The spectrum is a spectrum in a visible light region of the light that has passed through a glass base plate on which a silicon nitride film (50 nm), a silicon oxide film (200 nm), a silicon nitride film (200 nm), an organic resin film (2 xcexcm) and an ITO (50 nm) are laminated in the order from the bottom layer.
As apparent from the transmissivity spectrum, there appear interference phenomena that depend on refractive index differences between layers and film thicknesses. The interference causes the dispersion of wavelengths distribution of the transmitted light and a loss of the whole amount of the transmitted light. There exists a further problem that, because interference patterns vary in accordance with the dispersion of film thicknesses, the dispersion of colors occurs by respective display devices.
For resolving the aforesaid problems of the prior art, the following means is devised.
That is, a display device according to the present invention includes pixels disposed in a matrix on a transparent base plate, each pixel having an opening region, in which an electro-optic element for emitting light through the base plate is formed, and a non-opening region, in which a thin film transistor for driving the electro-optic element is formed, wherein the non-opening region has a first film structure including the thin film transistor, the opening region has a second film structure, which extends from the first film structure and exists between the electro-optic element and the base plate, the second film structure is different from the first film structure so as to adjust the light passing through the opening region.
In this case, the second film structure includes one or more than one films, and at least one of a number, thicknesses, refractive indices and light absorption rates of the films of the second film structure differs from those of the first film structure for adjustment of transmissivity or a color temperature of the light passing through the opening region.
Moreover, the second film structure differs from the first film structure so that the refractive indices of the second film structure become closer to a refractive index of the base plate than those of the first film structure.
Moreover, the base plate is made of glass, and the first film structure includes a silicon nitride film having a refractive index different from that of the glass, and further the silicon nitride film is removed from the second film structure.
Moreover, the first film structure includes at least a gate insulation film existing between an active layer and a gate electrode of the thin film transistor, an interlayer insulation film existing between the thin film transistor and its wiring, and a protection film for covering the thin film transistor, and at least one of the gate insulation film, the interlayer insulation film and the protection film is removed from the second film structure.
In this case, the gate insulation film or the interlayer insulation film is removed from the second film structure in a process of forming a contact hole to the gate electrode or the wiring.
In an aspect of the present invention, the thin film transistor has a bottom gate structure in which the active layer is superposed above the gate electrode with the gate insulation film inserted between the active layer and the gate electrode, and the wiring is formed above the active layer with the interlayer insulation film inserted between the wiring and the active layer, and further at least the gate insulation film or the interlayer insulation film is removed from the second film structure.
In another aspect of the present invention, the thin film transistor has a top gate structure in which the gate electrode is superposed above the active layer with the gate insulation film inserted between the gate electrode and the active layer, and the wiring is formed above the gate electrode with the interlayer insulation film inserted between the wiring and the gate electrode, and further at least the gate insulation film or the interlayer insulation film is removed from the second film structure.
In addition, the active layer is made of polycrystalline silicon.
Moreover, the protection film is made of a transparent organic resin film, and the second film structure includes the organic resin film as it is.
According to the present invention, a multilayer film structure formed in a non-opening region is not extended to an opening region as it is, but the physical structures of films are varied between the non-opening region and the opening region. That is, a second film structure existing in the opening region varies from a first film structure existing in the non-opening region for adjusting light passing through a pixel electrode. For example, a film layer the refractive index of which is greatly different from that of a glass base plate among a plurality of transparent films included in the first film structure may be removed from the second film structure for suppressing unnecessary reflection owing to multiplex interference to improve the transmissivity and the color temperature of the opening region.
According to the present invention, there is also provided a display device manufacturing method for forming an opening region including an electro-optic element for emitting light through a transparent base plate and a non-opening region including a thin film transistor for driving the electro-optic element in each pixel disposed in a matrix on the base plate, in which the method comprises the steps of: forming a first film body including the thin film transistor in the non-opening region, and forming a second film body extending from the first film body and existing between the electro-optic element and the base plate, wherein the second film body is different from the first film body for adjusting the light passing through the opening region.
According to the present invention, there is also provided a liquid crystal display device including pixels disposed in a matrix on a transparent base plate, each pixel having an opening region, where an electro-optic element for emitting light through the base plate is formed, and a non-opening region, where a thin film transistor for driving the electro-optic element is formed, the electro-optic element being made by holding a liquid crystal material between transparent electrodes opposing to each other, the electro-optic element emitting light that has entered from one surface side of the base plate to another surface side of the base plate, wherein the non-opening region has a first film structure including the thin film transistor, and the opening region has a second film structure extending from the first film structure and existing between the electro-optic element and the base plate, the second film structure varying from the first film structure so as to adjust the light passing through the opening region.
According to the present invention, there is also provided a liquid crystal display device manufacturing method for forming an opening region including an electro-optic element for emitting light through a transparent base plate and a non-opening region including a thin film transistor for driving the electro-optic element in each pixel disposed in a matrix on the base plate, the electro-optic element being made by holding a liquid crystal material between transparent electrodes opposing to each other, the electro-optic element emitting light that has entered from one surface side of the base plate to another surface side of the base plate, in which the method comprises the steps of: forming a first film body including the thin film transistor in the non-opening region, and forming a second film body extending from the first film body and existing between the electro-optic element and the base plate, the second film body varying from the first film body so as to adjust the light passing through the opening region.
According to the present invention, there is also provided an organic electroluminescent display device including pixels disposed in a matrix on a transparent base plate, each pixel having an opening region, where an electro-optic element for emitting light through the base plate is formed, and a non-opening region, where a thin film transistor for driving the electro-optic element is formed, the electro-optic element being made by holding an organic electroluminescent material between electrodes opposing to each other, the electro-optic element emitting light that has been generated by itself from one surface side of the base plate to another surface side of the base plate, wherein the non-opening region has a first film structure including the thin film transistor, and the opening region has a second film structure extending from the first film structure and existing between the electro-optic element and the base plate, the second film structure varying from the first film structure so as to adjust the light passing through the opening region.
According to the present invention, there is also provided an organic electroluminescent display device manufacturing method for forming an opening region including an electro-optic element for emitting light through a transparent base plate and a non-opening region including a thin film transistor for driving the electro-optic element in each pixel disposed in a matrix on the base plate, the electro-optic element being made by holding an organic electroluminescent material between electrodes opposing to each other, the electro-optic element emitting light that has been generated by itself from one surface side of the base plate to another surface side of the base plate, in which the method comprises the steps of: forming a first film body including the thin film transistor in the non-opening region, and forming a second film body extending from the first film body and existing between the electro-optic element and the base plate, the second film body varying from the first film body for adjusting the light passing through the opening region.
According to the present invention, because layers having a refractive index different from that of a transparent base plate are removed from an opening region as many as possible, multiplex interference is decreased and the transmissivity of a panel is improved. And, because the multiplex interference can be suppressed, deviations in color on manufacturing can be decreased. In addition, unnecessary reflection on a panel can be decreased.
When the layers having a refractive index different from that of the base plate is removed from the opening region, there is not required a new manufacturing process.
In particular, in a case of forming a thin film transistor having an active layer made of low temperature polysilicon, because a silicon oxide film is used as a gate insulation film and a silicon nitride film is used as an undercoat film or a passivation film (protection film) for preventing the thin film transistor from contamination from a glass plate, a lamination structure having a refractive index different from that of the glass is easily produced. In that case, there can be obtained a remarkable effect in promoting the transmissivity and preventing from coloring by removing silicon nitride films from opening regions selectively according to the present invention.